The long-term goal of this grant is to elucidate the molecules and mechanisms that sustain the accuracy of chromosome segregation. Such information is crucial for maintaining and improving human health, as inaccurate segregation causes aneuploidy and contributes to disorders like Down Syndrome and cancer. To facilitate the dissection of this and other complex biological processes, our laboratory has developed novel methods for precisely deleting or modifying genes in human somatic cells via homologous recombination. By combining these methods with high-resolution microscopy and chemical genetics, we have identified novel functions and regulation of the protein kinases Plk1 (Polo-like kinase 1) and Mps1, which are essential regulators of mitosis and cell division in all eukaryotes. Based on our preliminary findings, three aims are proposed for the next grant period: (1) to elucidate the effectors and mechanism of Plk1-dependent centrosome maturation; (2) to test Mps1's contribution to the cytosolic and kinetochore-dependent branches of the spindle assembly checkpoint (SAC); and (3) to identify the substrates of Mps1 that mediate the kinase's functions in SAC enforcement and chromosome bi-orientation. These studies will illuminate the mechanisms that regulate and sustain the high fidelity of chromosome segregation in humans. Such information is crucial for understanding how normal cells avoid aneuploidy and its adverse impacts on human health, and for developing novel treatments that specifically target aneuploidy in the context of disease.